JP2012104385A - Ceramic heater element manufacturing method and glow plug manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To dispense with a formed element body serving as a dummy for reducing influences of a reduction atmosphere in a ceramic heater element manufacturing method which involves calcining by a hot-press method a ceramic heater element having a heat generating resistor composed mainly of silicon nitride and tungsten carbide buried within an insulation base material made mainly of silicon nitride, thereby improving the productivity of ceramic heater elements.SOLUTION: A ceramic heater element manufacturing method involves calcining by a hot-press method a formed element body 18a having a base material part 21a which is composed mainly of silicon nitride powder and a resistor body forming part 22a, buried in the base material part, which is composed mainly of silicon nitride powder and tungsten carbide powder, to produce a ceramic heater element 18. The calcination includes an atmospheric pressurization process where, in a nitrogen atmosphere, the pressing pressure is made to be 20-40 MPa and the atmospheric pressure is made to be 0.2-1.0 MPa.

Description

  The present invention relates to a method for manufacturing a ceramic heater element and a method for manufacturing a glow plug.

  Conventionally, ceramic heater elements are used for glow plugs for promoting start-up of diesel engines, ignition heaters for burners, heaters for heating gas sensors, and the like. In the ceramic heater element, a heating resistor made of silicon nitride and tungsten carbide is embedded in a rod-like insulating base made of, for example, silicon nitride. Such a ceramic heater element is generally manufactured by firing by a hot press method.

  For firing by the hot press method, for example, an induction heating type firing furnace is used. The induction heating type firing furnace mainly includes a cylindrical mold that forms a firing chamber, a pair of press rods that are inserted into the mold and used for pressurization, and a coil that is disposed outside the mold and used for heating. Etc. In such a firing furnace, the vicinity of the outside of the mold is mainly heated by the high frequency from the coil, and the heat is transmitted to the inside to perform the heating. Members constituting the firing chamber such as a mold and a press rod are made of graphite for heating by high frequency and for securing strength at high temperature.

  In the manufacture of a ceramic heater element using such a firing furnace, first, an element molded body that becomes a ceramic heater element by firing is sandwiched between hot pressing molds to form a laminate. Usually, a laminated body is formed by laminating a plurality of hot pressing molds, and an element molded body is sandwiched between the hot pressing molds. Further, the hot pressing mold has a corrugated plate shape in which concave portions are formed in a row on substantially the entire surface, and is arranged so that the element molded body is spread over the entire concave portion.

  Such a laminate is placed in the firing chamber, heated by a coil provided in the firing furnace, and pressed by a pair of press bars. The element molded body is fired by such heating and pressurization to form a ceramic heater element (see, for example, Patent Document 1).

JP 2003-22889 A

  In the firing furnace, since the mold or the like constituting the firing chamber is made of graphite, the atmosphere inside thereof tends to be a strong reducing atmosphere. In particular, the element molded body arranged in the vicinity of the outer periphery of the hot press mold is easily exposed to such a strong reducing atmosphere. When the element compact is made of silicon nitride powder and tungsten carbide powder, it is exposed to a strong reducing atmosphere at a high temperature during firing, and silicon nitride and tungsten carbide react to produce brittle tungsten disilicide. As a result, the strength and energization durability of the ceramic heater element are reduced.

  For this reason, conventionally, a so-called dummy element molded body made of only silicon nitride is disposed near the outer periphery of a hot press mold so as to protect the regular element molded body disposed at the center. Yes. However, when a dummy element molded body is used, the number of regular element molded bodies that can be arranged in the hot press mold is reduced, and as a result, the number of regular ceramic heater elements obtained by firing is reduced. In addition, it is necessary to manufacture a dummy element molded body separately from the regular element molded body, and since the dummy ceramic heater element obtained by firing needs to be discarded, the number of manufacturing processes increases as a whole, Moreover, it is not necessarily preferable from the viewpoint of effective utilization of raw materials.

  The present invention has been made to solve the above-described problem, and an object of the present invention is to provide a manufacturing method that eliminates the need for a dummy element molded body and can improve the productivity of ceramic heater elements. . It is another object of the present invention to provide a manufacturing method that can improve the productivity of glow plugs by using a ceramic heater element manufactured by such a manufacturing method.

  The method for manufacturing a ceramic heater element of the present invention includes an element molding having a base molding part mainly made of silicon nitride powder and a resistor molding part mainly embedded in the base molding part and mainly made of silicon nitride powder and tungsten carbide powder. The present invention relates to a method for manufacturing a ceramic heater element by firing a body by a hot press method to obtain a ceramic heater element. The method for producing a ceramic heater element of the present invention is characterized in that the firing includes an atmosphere pressurizing step in which a pressing pressure is 20 to 40 MPa and an atmospheric pressure is 0.2 to 1.0 MPa in a nitrogen atmosphere.

  The glow plug manufacturing method of the present invention relates to a glow plug manufacturing method using a ceramic heater element manufactured by the above-described ceramic heater element manufacturing method. The method for manufacturing a glow plug according to the present invention includes a step of manufacturing a ceramic heater element used for a glow plug by the above-described method of manufacturing a ceramic heater element, and inserting the ceramic heater element into a metal outer cylinder that is a constituent member of the ceramic heater. And a step of inserting the metal outer cylinder into the tip of a cylindrical hole of a cylindrical metal shell, which is a constituent member of the glow plug, to form a glow plug.

  According to the method for manufacturing a ceramic heater element of the present invention, by performing an atmosphere pressurizing step of setting a predetermined press pressure and atmosphere pressure at the time of firing, the tungsten disilicide in the element molded body made of silicon nitride powder and tungsten carbide powder. Generation can be suppressed. This eliminates the need for a dummy element molded body and improves the productivity of the ceramic heater element. Further, by producing a glow plug ceramic heater element by such a ceramic heater element production method, the productivity of the glow plug can be improved.

The longitudinal section showing an example of the glow plug which has a ceramic heater element concerning the present invention. The figure which shows an example of the manufacturing method of an element molded object. The figure which shows an example of the integration method of an element molded object. The figure which shows an example of the baking method of an element molded object. The top view of the baking furnace for hot press shown in FIG. The figure which shows the relationship between the temperature and open porosity in an Example.

Hereinafter, embodiments of the present invention will be described.
The method for producing a ceramic heater element (hereinafter simply referred to as a heater element) of the present invention includes a base molded part mainly made of silicon nitride powder, and a resistor embedded in the base molded part and mainly made of silicon nitride powder and tungsten carbide powder. The present invention relates to a heater element manufacturing method in which an element molded body having a body molded portion is fired by a hot press method to form a heater element. The method for manufacturing a heater element of the present invention is characterized in that the firing includes an atmosphere pressurizing step in which a press pressure is 20 to 40 MPa and an atmospheric pressure is 0.2 to 1.0 MPa in a nitrogen atmosphere.

The following reaction formula (1) shows a reaction in which tungsten disilicide is generated from silicon nitride and tungsten carbide. The following reaction formulas (2) and (3) more specifically show the reaction of the reaction formula (1). In the reaction in which tungsten disilicide is generated from silicon nitride and tungsten carbide, silicon nitride is first thermally decomposed into silicon and nitrogen as shown in reaction formula (2), and this silicon is shown in reaction formula (3). It proceeds by reacting with tungsten carbide.
Si ₃ N ₄ + WC → WSi ₂ + SiC + 2N ₂ ↑ (1)
Si ₃ N ₄ → 3Si + 2N ₂ ↑ (2)
3Si + WC → WSi ₂ + SiC (3)

  A reaction in which tungsten disilicide is generated from silicon nitride and tungsten carbide, particularly a reaction in which silicon nitride is thermally decomposed into silicon and nitrogen, is likely to proceed under a high temperature and a strong reducing atmosphere as in a firing furnace. According to the atmosphere pressurization step, even under a high temperature and strong reducing atmosphere, by performing the atmosphere pressurization at a pressure higher than the conventional pressure simultaneously with the press pressurization, the nitrogen concentration in the atmosphere is increased, and the reaction formula Thermal decomposition of silicon nitride shown in (2) can be suppressed.

  As a result, the reaction of silicon and tungsten carbide shown in the following reaction formula (3) can be suppressed, and the formation of tungsten disilicide can be suppressed. As a result, it is possible to place a regular element molding near the outer periphery of a hot press mold, which has conventionally been provided with a dummy element molding because it is easily exposed to a reducing atmosphere, thereby producing heater elements. Can be improved.

  When the atmosphere pressure (nitrogen partial pressure) of the nitrogen atmosphere in the atmosphere pressurization step is less than 0.2 MPa (2 atm), the atmosphere pressure is not sufficiently high, that is, the nitrogen concentration in the atmosphere is not sufficiently high. The thermal decomposition of silicon nitride shown in the reaction formula (2) cannot be suppressed. On the other hand, if the atmospheric pressure is 1.0 MPa (10 atm), the thermal decomposition of silicon nitride shown in the reaction formula (2) can be sufficiently suppressed. This is not preferable because the effect is not improved, the manufacturing cost is increased, and the burden on equipment such as a firing furnace is increased.

  Moreover, when the press pressure in an atmosphere pressurization process is less than 20 MPa, since the press pressure is not sufficiently high, the thermal decomposition reaction of silicon nitride represented by the reaction formula (2) cannot be suppressed. On the other hand, if the pressing pressure is 40 MPa, the thermal decomposition of silicon nitride shown in the reaction formula (2) can be sufficiently suppressed, and if it exceeds this, the members constituting the firing chamber and the hot pressing mold are damaged. It becomes easy.

  The atmosphere pressurization step, particularly the atmosphere pressurization, is preferably started when the temperature during the temperature increase is 1600 to 1700 ° C. When the temperature is 1600 ° C. or higher, the surface of the element molded body is sufficiently fired, and the open pores disappear. By performing atmospheric pressure process, particularly atmospheric pressure, on such a state, it is possible to suppress the high pressure atmosphere from entering the inside of the element molded body, and the strength is lowered by the residual pores made of this high pressure atmosphere. Can be suppressed. On the other hand, when the temperature exceeds 1700 ° C., thermal decomposition of silicon nitride shown in the reaction formula (2) proceeds, and as a result, it becomes difficult to suppress the formation of tungsten disilicide. By setting the start temperature of the atmosphere pressurization step, particularly the atmosphere pressurization to 1700 ° C. or less, thermal decomposition of silicon nitride can be effectively suppressed, and as a result, formation of tungsten disilicide can be suppressed.

  In the firing in the present invention, first, the temperature is first raised in a nitrogen atmosphere at an atmospheric pressure of about 0.1 MPa (1 atm). During the temperature increase, atmospheric pressure is applied at 1600 to 1700 ° C. so that the atmospheric pressure becomes 0.2 MPa or more. Further, the firing is performed by raising the temperature to a desired firing temperature, for example, 1700 to 1850 ° C., while performing such an atmosphere pressurizing step. In addition, it is preferable to perform an atmosphere pressurization process until it reaches | attains a desired calcination temperature, and also the maintenance at a desired calcination temperature is complete | finished.

  The pressurization in the atmosphere pressurization process does not necessarily need to be started simultaneously with the atmosphere pressurization. For example, the pressurization is started before the atmosphere pressurization process, and this pressurization is maintained as it is in the atmosphere pressurization process. Also good. Normally, it is preferable that pressurization is completed before the shrinkage start temperature (liquid phase generation start temperature) of the element molded body, and the pressurization is completed by 1450 ° C, for example, although it depends on the components of the sintering aid. It is preferable to maintain this in the subsequent temperature increase and atmospheric pressure step.

  As a heater element manufactured by such a manufacturing method, the insulating base formed by firing the base molding portion is mainly composed of silicon nitride, and the heating resistor formed by firing the resistor molding portion is silicon nitride and tungsten carbide. If it has as a main component, there will be no restriction | limiting in particular.

  In addition to silicon nitride, the insulating substrate can contain a sintering aid, a conductive ceramic constituting a heating resistor, and the like. For example, it contains 1 to 10% by mass in terms of oxide of at least one element selected from the group of elements 3A, 4A, 5A, 3B (for example, Al), and 4B (for example, Si) in the periodic table, and Mg. can do. These components are mainly contained in the form of complex oxides such as oxides or silicates. When the content of these components is less than 1% by mass, it is difficult to obtain a dense sintered body. When the content exceeds 10% by mass, the strength, toughness or heat resistance is insufficient.

  Moreover, when using rare earth elements, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu can be used. Among these, Tb, Dy, Ho, Er, Tm, and Yb can be suitably used because they promote the crystallization of the grain boundary phase and improve the high temperature strength. These rare earth elements can be contained in an amount of 1 to 15% by mass in terms of oxide.

  The conductive ceramic can be contained in an amount of 5% by mass or less, for example, when the entire insulating substrate is 100% by mass. By setting it as such content, the thermal expansion difference with a heating resistor can be reduced. The content of silicon nitride is preferably 80% by mass or more when the entire insulating substrate is 100% by mass.

  In addition to silicon nitride and tungsten carbide, the heating resistor can contain a sintering aid similar to that of the insulating substrate. The content of the sintering aid or the like is preferably 10% by mass or less when the entire heating resistor is 100% by mass, for example. Moreover, 45-65 mass% of tungsten carbide is preferable and 50-60 mass% is more preferable, when silicon nitride and tungsten carbide make these total amount 100 mass%.

  As such a heater element, a rapid heating type heater element is particularly preferable. In the rapid heating type heater element, the heat generating portion of the heat generating resistor has a smaller diameter than the lead portion, and the insulating base portion where the heat generating portion is located is thicker than the insulating base portion where the lead portion is located. ing. In general, the shrinkage rate when firing an insulating substrate is larger than that of a heating resistor. Therefore, when such a product is manufactured by a hot press method, the heating portion is positioned as compared with the insulating substrate where the lead portion is positioned. The shrinkage of the insulating substrate in the portion to be increased increases, and as a result, a gap is formed with the hot press mold. For this reason, the pressing pressure on the part where the heat generating part is located is not sufficient, tungsten disilicide is easily generated, and the nitrogen atmosphere is sealed at the interface part between the insulating substrate and the heat generating resistor, particularly the interface part between the heat generating part. Easy to be.

According to the manufacturing method including the atmosphere pressurizing step, even in such a rapid temperature rising type heater element, the generation of tungsten disilicide or the like can be suppressed, and the strength and energization durability can be made sufficient. As a rapid heating type heater element, in order to ensure crack resistance when energized, the cross-sectional area of the minimum diameter of the heating resistor is set to 0.25 mm ² or more, and rapid heating performance is ensured. In addition, the cross-sectional area of the portion having the maximum diameter may be twice or more the cross-sectional area of the portion having the minimum diameter. Usually, the minimum diameter exists in any part of the heat generating part, and the maximum diameter exists in any part of the lead part. Sectional area of the portion to be the smallest diameter is preferably 0.28~1.2mm ^2, the cross-sectional area of the portion having the maximum diameter 2.4 to 8.1 times the cross-sectional area of the portion where the minimum diameter is preferred.

  Hereinafter, the heater element used for the glow plug will be specifically described by way of example. First, the structure of a glow plug or the like will be described.

  The glow plug 10 includes a ceramic heater 11 and a cylindrical metal shell 12 that holds the rear end portion of the ceramic heater 11 inside. On the outer peripheral surface of the metal shell 12, a screw portion 121 as an attachment portion for fixing the glow plug 10 to an engine block (not shown) is formed, and the rear end portion is used for tightening when fixing to the engine block. A tool engaging portion 122 having a hexagonal cross section is formed to engage the tool.

  Inside the metal shell 12, a columnar metal shaft 13 for supplying electric power to the ceramic heater 11 from the rear end side is insulated from the metal shell 12 with its rear end protruding from the metal shell 12. Is arranged in. At the rear end of the metal shell, an O-ring 14 made of an insulating material such as fluoro rubber and an insulating bush 15 made of resin such as nylon are disposed around the metal shaft 13. The insulating bush 15 has a flange portion 151 projecting radially outward on the rear end side of the insulating bush 15 and a small-diameter portion 152 having a cylindrical shape narrower than the flange portion 151 on the front end side. The insulating bush 15 has the metal shaft in such a manner that the small diameter portion 152 is fitted into the rear end portion of the cylindrical hole of the metal shell 12 so that the front surface of the flange portion 151 contacts the rear end of the metal shell 12. 13 around. The O-ring 14 is disposed so as to be pressed against the front end surface of the small-diameter portion 152 and to be in contact with the cylindrical hole of the metal shell 12 and the metal shaft 13, and the airtightness between the inside and the outside at the rear end portion of the glow plug 10. Is maintained.

  A terminal fitting 17 is fitted into the rear end portion of the metal shaft 13 extending rearward of the metal shell 12. The terminal fitting 17 is fixed to the outer peripheral surface of the metal shaft 13 in a conductive state by a caulking portion 17a in the radial direction.

  The ceramic heater 11 includes a heater element 18 and a metal outer cylinder 19 that holds the heater element 18 inside such that the tip portion protrudes. The metal outer cylinder 19 has a cylindrical shape as a whole, and is formed to be relatively thick through a small diameter portion 191 formed relatively thin on the front end side and a tapered portion 192 that expands on the rear end side of the small diameter portion. Further, an engaging portion 194 having an outer diameter substantially the same as the cylindrical hole of the metal shell 12 and having a smaller diameter than that of the large diameter portion 193 is provided on the rear end side of the large diameter portion 193. The engaging portion 194 is inserted into the front end of the cylindrical hole of the metal shell 12, the rear end facing surface of the large diameter portion 193 and the front end surface of the metal shell 12 are brought into contact, and the contact portion is laser welded all around. The metal outer cylinder 19 and the metal shell 12 are fixed.

  The heater element 18 has a rod shape in which a heating resistor 22 mainly composed of silicon nitride and tungsten carbide is embedded in an insulating base 21 mainly composed of silicon nitride. The heating resistor 22 has a U-shaped heating part 23 disposed on the front end side of the heater element 18 and a pair of straight lines connected to both ends and extended along the axial direction of the heater element 18. And a lead portion 24. One of the lead portions 24 is formed with a ground energizing terminal portion 25 that branches in the radial direction and is exposed to the side surface of the heater element 18, and is electrically connected to the metal shell 12 through a metal outer cylinder 19 that press-fits the heater element 18. Connected. On the other side of the lead portion 24, a power supply side energization terminal that branches off from the linear portion and is exposed to the side surface of the heater element 18 in a shape similar to the ground energization terminal portion 25 behind the ground energization terminal portion 25. A portion 26 is formed. A cylindrical ring member 16 joined to the tip of the metal shaft 13 is fitted on the rear end of the heater element 18, whereby the power supply side energizing terminal portion 26 is electrically connected to the metal shaft 13. As a result, the heat generating portion 23 folded back in the U shape of the heat generating resistor 22 generates resistance heat.

Such a heater element 18 can be manufactured as follows.
First, as shown in FIG. 2, a resistor molding portion 22 a that becomes a heating resistor 22 is manufactured by injection molding of a molding material. The resistor molding part 22 a has a heating element molding part 23 a that becomes the heating part 23 and a lead molding part 24 a that becomes the lead part 24. The molding material is obtained by melting and fluidizing a compound obtained by kneading a raw material powder for a resistor in which a sintering aid powder is blended with a silicon nitride powder and a tungsten carbide powder and an organic binder.

  Separately, the raw material powder for an insulating substrate is press-molded to produce a pair of substrate molding portions 21a formed in separate upper and lower bodies. On the mating surface of the base molding portion 21a, concave portions 27 having shapes corresponding to the resistor molding portions 22a are formed.

  Next, the base molding portion 21 a is fitted on the mating surface so that the resistor molding portion 22 a is accommodated in the recess 27. And as shown in FIG. 3, it accommodates in the metal mold | die 31 and press-molds with a pair of punch 32, and it is united and it is set as the element molded object 18a. The element molded body 18a is calcined at about 600 to 800 ° C. in order to remove the binder component. The element molded body 18a subjected to the calcination is fired by a hot press method having an atmosphere pressurizing step to obtain the heater element 18.

FIG. 4 is a cross-sectional view showing an example of a firing method of the element molded body 18a.
Firing is performed using, for example, an induction heating type hot press firing furnace (hereinafter simply referred to as a firing furnace) 40. The firing furnace 40 is arranged in a cylindrical mold 41 such as a cylindrical shape, a press rod 42 inserted into the mold 41 and applying a pressing pressure to the element molded body 18a, and a side surface portion of the mold 41. A coil 43 and the like for heating with application of.

  A partition plate 46 that defines the firing chamber 45 is disposed in the mold 41. The partition plate 46 has a plate shape extending in the pressure axis direction, and for example, as shown in FIG. 5, four plates are arranged in a square shape in a plan view to form a cylindrical shape. A gap adjusting plate 47 for filling a gap between the partition plate 46 and the mold 41 is disposed outside each partition plate 46. The gap adjusting plate 47 has an arcuate shape whose outer surface matches the shape of the mold 41, and a flat shape whose inner surface matches the partition plate 46. The members constituting the firing chamber 45, that is, the mold 41, the press bar 42, the partition plate 46, and the gap adjusting plate 47 are generally made of the same material and made of graphite.

  The element molded body 18 a is disposed in the firing chamber 45 so as to be sandwiched between the hot press molds 48. The lowermost (first stage) hot pressing mold 48 has a flat bottom surface and a plurality of concave portions in which a plurality of element molded bodies 18a are arranged on the top surface. In the lowermost hot press mold 48, for example, a carbon sheet (not shown) is disposed, and then the element molded body 18 a is disposed. The carbon sheet is used to fill a gap between the concave portion of the hot press mold 48 and the element molded body 18a and apply an appropriate pressing pressure to the element molded body 18a.

  On the lowermost element molded body 18a, a second-stage hot pressing mold 48 is disposed via a carbon sheet. In the second and subsequent hot pressing molds 48, concave portions in which a plurality of element molded bodies 18a are arranged are formed in rows on both the lower surface and the upper surface. In this way, the hot-press molds 48 and the element molded bodies 18a are alternately laminated, and the uppermost stage has a row of concave portions in which a plurality of element molded bodies 18a are arranged only on the lower surface. A mold 48 is stacked. Usually, the hot press mold 48 is made of graphite.

  The laminated body in which the hot pressing molds 48 and the element molded bodies 18 a are alternately laminated in this way is disposed in the firing chamber 45 of the firing furnace 40 so as to be sandwiched between the graphite plate members 49. A high frequency is applied from the coil 43 to heat the mold 41 and pressurization is performed by a pair of press rods 42 to perform firing.

  The firing is first performed in a nitrogen atmosphere at an atmospheric pressure of about 0.1 MPa (1 atm). During the temperature rise, for example, press pressurization with a press pressure of 20 to 40 MPa is started by, for example, the shrinkage start temperature (liquid phase generation start temperature) of the element molded body 18a, for example, 1450 ° C., and this state is maintained. Further, during the temperature increase, the atmosphere pressurization step, particularly the atmosphere pressurization with the atmosphere pressure of 0.2 to 1.0 MPa is started at 1600 to 1700 ° C., and this state is maintained. Thereafter, the temperature is continuously increased, and when it reaches a desired firing temperature, for example, 1700 to 1850 ° C., it is maintained for about 1 to 2 hours, and then cooled. In addition, it is not always necessary to perform an atmosphere pressurization process at the time of cooling. What is taken out from the hot pressing mold 48 is subjected to polishing of the outer peripheral surface by centerless polishing, end surface polishing at both ends, and further anti-spherical polishing at the front end to form the heater element 18.

  According to the manufacturing method having such an atmosphere pressurizing step, since it is possible to suppress the formation of tungsten disilicide, conventionally, a hot element in which a dummy element molded body is disposed because it is easily exposed to a reducing atmosphere. The regular element molded body 18a can also be disposed near the outer periphery of the pressing mold 48, and the productivity of the heater element 18 can be improved.

  Hereinafter, the production method of the present invention will be described more specifically with reference to examples.

Example 1
An insulating component powder was prepared by blending 85% by mass of silicon nitride powder having an average particle size of 1.0 μm, 10% by mass of Yb ₂ O ₃ powder as a sintering aid and 5% by mass of SiO ₂ powder. The insulating component powder 45 wt% and the WC powder 55 wt% were wet mixed in a ball mill for 24 hours and dried to obtain a mixed powder. A predetermined amount of a binder was added to the mixed powder and kneaded for 4 hours with a kneader. The kneaded material was pelletized and charged into an injection molding machine to form a resistor molding portion 22a. The resistance-forming section 22a is the maximum cross-sectional area of the portion to be a minimum diameter 1.0 mm ^2, the cross-sectional area of the portion having the maximum diameter to 2.8 mm ² (minimum diameter portion when the heating resistor 22 The cross-sectional area ratio of the diameter portion was 2.8) (for rapid temperature rising type).

Meanwhile, 83% by mass of silicon nitride powder having an average particle size of 0.6 μm, 10% by mass of Yb ₂ O ₃ powder as a sintering aid, 5% by mass of SiO ₂ powder, and ₂ % by mass of MoSi ₂ powder were blended, The mixture was added, wet mixed for 20 hours, and granulated by spray drying. The granulated powder was press-molded to produce a pair of base body molded portions 21 a having recesses 27.

  After the base molding part 21a was fitted so as to accommodate the resistor molding part 22a in the recess 27, it was accommodated in the mold 31 and press-molded by a pair of punches 32 to be integrated into the element molding 18a. Further, the element compact 18a was calcined at 600 ° C. in a nitrogen atmosphere in order to remove the binder component.

  Then, as shown in FIG. 4, a plurality of element molded bodies 18a are arranged in the firing furnace 40 so as to be sandwiched between hot pressing molds 48, and the atmospheric pressure is increased to 0.1 MPa (1 atm) in a nitrogen atmosphere. The temperature was started and firing was performed at 1720 ° C or 1800 ° C. At this time, pressurization was started up to 1450 ° C., which is lower than the shrinkage start temperature (liquid phase generation start temperature) of the constituent materials, and this state was maintained. The press pressure was 15 to 40 MPa. In addition, as an atmosphere pressurizing step, an atmosphere pressurization was started at 1650 ° C., and the atmosphere pressure of the nitrogen atmosphere was 0.1 to 1.0 MPa, and this state was maintained. In addition, the press pressurization in an atmosphere pressurization process utilized the said press pressurization as it was.

Of the plurality of heater elements 18 thus manufactured, those obtained from the vicinity of the outer periphery of the hot press mold 48, whether or not tungsten disilicide (WSi ₂ ) was generated, bending strength, and current durability. Was evaluated. Tables 1 and 2 show the results when the firing temperatures are 1720 ° C and 1800 ° C, respectively.

  Note that the presence or absence of tungsten disilicide was evaluated by the ICP emission method. Further, the bending strength was measured by a three-point bending strength according to JIS R 1601. The span at this time was 12 mm, and the crosshead speed was 0.5 mm / min.

  Furthermore, the current-carrying durability was evaluated as follows. First, a voltage of 1000 ° C. is applied after 1 second of energization, and then the voltage is maintained until it reaches 1400 ° C. When 1400 ° C. is reached, the energization is turned off and forced cooling is performed with air for 30 seconds. This cycle was repeated, and the number of cycles until the resistance value of the heating resistor changed by 10% or more with respect to the initial resistance value was measured. The resistance value was measured up to 10,000 cycles.

As shown in Tables 1 and 2, when the pressing pressure is 20 MPa or more and the atmospheric pressure is 0.2 to 1.0 MPa, the formation of tungsten disilicide (WSi ₂ ) is suppressed, and at least 10,000 cycles or more. It was confirmed that a current-carrying durability and a bending strength of 800 MPa or more can be obtained.

(Example 2)
A heater element 18 was manufactured basically in the same manner as in Example 1 except that the start temperature of the atmosphere pressurization step (atmosphere pressurization) was changed to 1470 to 1710 ° C. The firing temperature was 1720 ° C., the atmospheric pressure in the atmospheric pressure step was 0.5 MPa, and the press pressure was 30 MPa. In addition, with respect to the heater element 18 thus obtained, the presence or absence of tungsten disilicide (WSi ₂ ) generation, the presence or absence of interface pores (the pores at the interface between the insulating substrate and the heating resistor), the current durability, The bending strength was evaluated. The presence or absence of interface pores was evaluated by observation with a scanning electron microscope (SEM) in a field of view with a magnification of 3000 times. The results are shown in Table 3.

Next, the heater element 18 was manufactured in the same manner as in Example 1 by changing the cross-sectional area ratio of the maximum diameter portion to the minimum diameter portion of the resistor molding portion 22a. The resistor molding portion 22a is such that the cross-sectional area of the minimum diameter portion is constant at 1.0 mm ² when the heating resistor 22 is formed by firing, and the cross-sectional area ratio of the maximum diameter portion to the minimum diameter portion is constant. 1-6. The firing temperature was 1800 ° C., the atmospheric pressure in the atmospheric pressure step was 0.5 MPa, and the press pressure was 35 MPa. The heater element 18 thus obtained was evaluated for the presence or absence of residual pores (pores due to penetration of a high-pressure atmosphere) and the bending strength. The presence or absence of residual pores was evaluated by observation with a scanning electron microscope (SEM) in a field of view with a magnification of 3000 times. The results are shown in Table 4.

  Table 5 shows the surface open porosity when the element molded body 18a is fired to 1550 to 1700 ° C., and atmospheric pressurization with an atmospheric pressure of 0.5 MPa on those having such open porosity. Indicates the presence or absence of residual pores. FIG. 6 shows the relationship between the temperature shown in Table 5 and the open porosity.

The resistor molding portion 22a has a minimum sectional area of 1.0 mm ² when the heating resistor 22 is formed, and the sectional area ratio of the maximum diameter portion to the minimum diameter portion is 2.4. Doubled. Further, the open porosity was calculated by the following formula when the dry weight of the fired body was W ₁ , the weight in water was W ₂ , and the air weight when the excess moisture on the surface was wiped off with a compress was W ₃ .
Open porosity (%) = [(W ₃ −W ₁ ) / (W ₃ −W ₂ )] × 100

  As shown in Tables 4 and 5 and FIG. 6, the open porosity disappears when the temperature is 1600 ° C. or higher, and by starting the atmosphere pressurization at such a temperature, the sectional area ratio of the heating resistor 22 is reduced. It can be seen that the generation of residual pores can be effectively suppressed even when the size is large. In addition, as shown in Table 3, the generation of interface pores can be suppressed by setting the atmospheric pressure start temperature to 1500 ° C. or higher, and the formation of tungsten disilicide by reducing the temperature to 1700 ° C. or lower. Can be suppressed. From the above results, it can be seen that the starting temperature of the atmosphere pressurization (atmosphere pressurization step) is preferably 1600 to 1700 ° C.

DESCRIPTION OF SYMBOLS 10 ... Glow plug 11 ... Ceramic heater 12 ... Main metal fitting 19 ... Metal outer cylinder 18 ... Ceramic heater element (18a ... Element molded object)
21 ... Insulating base (21a ... Base molding part)
22 ... exothermic resistor (22a ... resistor molding part)

Claims (4)

An element molded body having a base molding portion mainly made of silicon nitride powder and a resistor molding portion embedded in the base molding portion and mainly made of silicon nitride powder and tungsten carbide powder is fired by a hot press method to be a ceramic heater. A method of manufacturing a ceramic heater element as an element,
The method for producing a ceramic heater element, wherein the firing includes an atmosphere pressurizing step in which a pressing pressure is 20 to 40 MPa and an atmospheric pressure is 0.2 to 1.0 MPa in a nitrogen atmosphere. The ceramic heater element has an insulating base formed by firing the base molding portion and a heating resistor formed by firing the resistor molding portion, and the heating resistor has a minimum diameter. ^2. The ceramic heater element according to claim 1, wherein the cross-sectional area of the portion is 0.25 mm ² or more and the cross-sectional area of the portion having the maximum diameter is twice or more the cross-sectional area of the portion having the minimum diameter. Production method.   The method for manufacturing a ceramic heater element according to claim 1 or 2, wherein the atmosphere pressurizing step is started at 1600 to 1700 ° C. A step of manufacturing a ceramic heater element used for a glow plug by the method of manufacturing a ceramic heater element according to claim 1;
The ceramic heater element is inserted into a metal outer cylinder that is a constituent member of the ceramic heater, and the metal outer cylinder is inserted into a cylindrical hole end of a cylindrical metal shell that is a constituent member of the glow plug to form a glow plug. And a process for producing a glow plug.

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